Method and apparatus for estimating the condition of a coating on an underground pipeline

Abstract

Methods are provided for reducing interference from stray currents in buried pipelines/metal structures during MEIS testing or other current-sensing applications in the pipeline. Methods are also provided for measuring bulk complex electrical impedance between a buried pipe and the soil, thereby rendering an indication of the quality of the anti-corrosive coating. Methods are also provided for measuring the complex propagation constant of AC voltages propagating along an attenuative pipeline. This information is useful for assessing the general condition of the anti-corrosive coating involved, or to enhance MEIS inspection of the pipeline. Methods are also provided for enhancements to MEIS testing, including (a) canceling magnetometer offset effects associated with the Earth's magnetic field after the magnetometer is positioned for measurement, (b) implementing a separate sensing connection to the pipe so as to avoid interference from voltage loss in the pipe feed-line connection, (c) providing a power amplifier to excite the pipe with large-amplitude signals.

1. A method for estimating a condition of a coating on an underground metal structure using bulk structure-to-soil spectroscopy, comprising:
 providing an AC voltage between a ground-return electrode and an injection end point of the structure via a feed line at a plurality of test frequencies; 
 positioning a reference electrode proximate said structure; 
 positioning a magnetometer adjacent to the feed line to sense all currents being provided to the structure; 
 receiving a first voltage potential representing a difference between the voltage at the injection end point and the reference electrode; 
 receiving a second voltage potential representing a difference between a voltage at the injection end point and the ground-return electrode; and 
 computing, from the first voltage potential and the magnetometer-measured currents, a first net impedance between the structure and soil at the plurality of frequencies, thereby defining a first impedance spectrum; 
 computing, from the second voltage potential and magnetometer measured currents, a second net impedance between the structure and the ground-return electrode at the plurality of frequencies, thereby defining a second impedance spectrum; 
 determining the difference between the first and second net impedances and/or determining the difference between the first and second impedance spectra, said differences providing indicia of the earthing resistance of the ground-return electrode; and 
 analyzing the first, second or both impedance spectra to assess the general or bulk condition of the coating on the structure under test. 
 
     
     
       2. The method of  claim 1 , wherein said estimating a condition of a coating on an underground metal structure comprises: estimating a condition of a coating on a pipeline buried in soil. 
     
     
       3. The method of  claim 2 , further comprising:
 receiving a third voltage value representing a difference between the voltage at a next or down-pipe electrical access point on the structure; and 
 computing amplitude, attenuation and phase shift metrics of the third voltage values relative to the injection end point voltage at each test frequency, wherein said metrics define a propagation constant for the pipeline. 
 
     
     
       4. The method of  claim 3 , further comprising estimating from the metrics a coating condition of the structure as between adjacent electrical access points. 
     
     
       5. The method of  claim 3 , further comprising determining, from the metrics, actual pipe-to-reference electrode complex voltage or complex voltage spectrum at any test location between adjacent electrical access points on the pipe. 
     
     
       6. The method of  claim 5 , further comprising conducting MEIS testing at any test location between the adjacent electrical access points utilizing the actual pipe-to-reference electrode complex voltage for that location as calculated from the amplitude and phase shift metrics. 
     
     
       7. The method of  claim 1 , further comprising the step of calibrating the magnetometer at a predetermined number of the test frequencies.